Before changing the hoses did you ever squeeze the old hoses and hear a crackling sound? If so, that would be scale build up on the inside of the hoses. Unfortunately that crackling sound is the sound of it breaking apart and forming small to large chunks that can get caught in the radiator and plug it. It's possible that you're radiator is just not allowing the flow needed at all times. WHen I bought my Frontier I had an overheat issue. It would only overheat at highway speeds or going up large hills (mountain passes really). Basically the engine was working harder and producing more heat than the radiator could handle, but it would be fine around town. I swapped out the water pump, the fan clutch, the thermostat. During this time I had to drive with the windows down and the heater turned all the way up, which brought the temp to where it needed to be. Nothing helped until I replaced the radiator. Haven't had an issue since.
If you do replace the radiator I recommend getting one meant for the Supercharged Xterra. It will fit just fine, but they have a double core rather than a single, which gives it a little more cooling capacity.
